Array indices must be positive integers or logical values

I'm trying to do the Bairstow method but the fuctions to get delta r and delta s mark me as an error

    coeff= length(fx);
    r=r0;
    s=s0;
    ErrR=100;
    ErrS=100;
    while ErrR>tol || ErrS>tol
        fx=fx(coeff:-1:1);
        b(coeff)= fx(coeff)
        b(coeff-1)= fx(coeff-1)+r*b(coeff);
        for rs=(coeff-2:-1:1)
            b(rs)= fx(rs)+r*b(rs+1)+s*b(rs+2);
        end
        c(coeff)= b(coeff);
        c(coeff-1)= b(coeff-1)+r*c(coeff);
        for rs=(coeff-2:-1:2)
            c(rs)= b(rs)+r*c(rs+1)+s*c(rs+2);
        end
        deltaR= (b(0)*c(3)-b(1)*c(2))/((c(2))^2-c(1)*c(3))
        deltaS= (b(1)*c(1)-b(0)*c(2))/((c(2))^2-c(1)*c(3))
        r= deltaR+r;
        s= deltaS+s;

Answer 1

[Jim Riggs]

Matlab does not allow an index value of zero.

deltaR = (b(0) ...
    

similar in deltaS.